The invention relates to a method for producing the image of a surface of a specimen to be examined with a resolution which is better than 1 pm laterally to the surface and better than 100 nm perpendicularly to the surface, with a scanning probe scanning the surface point by point and the distance between the scanning probe and the specimen surface being modulated at each scanning point, thus leading to a force-time curve. The invention also provides a scanning probe microscope for performing such a method.
A large number of methods for imaging the surfaces of specimen pieces by means of scanning probes have already been described in the state of the art.
One possibility for examining a specimen surface and for producing a surface image by means of a scanning probe is that the scanning probe is brought into contact with the surface of the specimen and the surface of the specimen is then scanned. Such an imaging method is known as “contact-mode” and is used for example for determining the topography and the local friction. With respect to the “contact-mode method”, reference is hereby made to the following publications:    Maivald P, Butt H-J, Gould S A C, Prater C B, Drake B, Gurley J A, Elings V B and Hansma P K (1991), Using force modulation to image surface elasticities with the atomic force microscope, Nanotechnology, 2, 103-105, and    Marti O and Colchero J, 1992, “Reibungsmikroskopie” (Frictional Microscopy), “Phys. Rafter” 48, 107,the disclosure of which shall both be fully included in the present application by reference.
The disadvantage of this imaging method is that when moving the scanning probe which is in contact with the surface of the specimen piece, shearing forces will occur which deform the surface of soft specimens such as polymeric or biological systems or can even destroy the same.
In order to protect a surface from deformation or destruction it is advantageous to examine the specimen surface with the help of a method in which the scanning probe is not in contact with the same. This method is generally known in literature as “non-contact mode”. It is a method with which a destruction of the specimen surface can be excluded entirely. One disadvantage of this method is however that the resolution decreases with increasing distance between scanning probe and specimen surface and no mechanical specimen properties can be examined.
Reference is hereby made with respect to the “non-contact mode”, the disclosure of which are both hereby fully included in the present application by reference:    Martin Y, Williams C C and Wickramsinghe H G. (1987), Atomic force microscope-force mapping and profiling on a sub 100-A scale, J. App. Phys., 61, 4723;    Sarid D, Ruskell T G, Workman R K and Chen D, 1996, J. Vac. Sci. Technol. B, 14, 864-7.
A method which allows the examination of soft specimen surfaces such as those of polymers but which on the other hand still offers sufficient information on the specimen surface is the so-called “intermediate-contact-mode” method, in which a scanning probe can be made to oscillate close to its natural frequency. The oscillating scanning probe is moved towards the specimen until it touches the specimen surface. The phase shift between the free oscillation in air and the oscillation when the scanning probe approaches the surface depends on the elastic-viscous properties of the probe and the adhesive potential between specimen and scanning probe. In this way it is possible to determine the elastic properties.
Reference is hereby made with respect to the “intermediate-contact-mode” method to the following:    Spatz J, Sheiko S, -Moller M, Winkler R, Reineker P and Marti O, (1995), Forces affecting the substrate in the resonant tapping force microscopy, Nanotechnology, 6, 40-44;    Digital Instruments, Incorporated, U.S. Pat. No. 5,412,980 (1995), Tapping atomic force microscope;    Digital Instruments, Incorporated, U.S. Pat. No. 5,519,212 (1996), Tapping atomic force microscope with phase or frequency detection,which disclosures are hereby fully included in the present application by reference.
The “intermediate-contact-mode” method comes with the disadvantage that the two variables, amplitude and phase shift, depend on a plurality of variables, so that a simple allocation to a physical variable is not possible.
These disadvantages can be overcome in such a way that the entire force-path or force-time curve is absorbed when the scanning probe approaches the specimen surface. This curve comprises the entire information of the interaction forces between scanning probe and specimen and allows a precise definition of the elastic-viscous properties and the adhesive forces.
Concerning this method reference is hereby made to:    Radmacher M, Cleveland J P, Fritz M, Hansma H G and Hansma P K, (1994) Mapping interaction forces with the atomic force microscope, Biophys. J, 66, 2159-65;    Radmacher M, Fritz M, Cleveland J P, Walters D A and Hansma P K, (1994 Imaging adhesion forces and elasticity of lysozyme adsorbed on mica with the atomic force microscope, Langmuir 10, 3809-14;    Van der Werf K O, Putman C A J, Groth B G and Greve J (1994), Adhesion force imaging in air and liquid by adhesion mode atomic force microscopy, Appl. Phys. Left, 65, 1195-7;    Mizes H A, Loh K-G, Miller R J D, Ahujy S K and Grabowskie E F (1991), Submicron probe of polymer adhesion with atomic force microscopy; dependence on topography and material inhomogeneities, Appl. Phys. Lett. 59, 2901-3Which disclosures are hereby fully included in the present application by reference.
The disadvantageous aspect in this method is that the scanning speed for recording an image is very low.
In order to increase this speed a new method was developed, the so-called “pulsed-force-mode” microscopy. In “pulsed-force-mode” (PFM) microscopy, the scanning probe is made to oscillate periodically in the z direction, i.e. the perpendicular direction relative to the specimen surface, and the force-time curve, which is an image of the force-path curve, is recorded and certain parameters of this force-time curve are evaluated with the help of analog circuits such as trigger circuits in order to obtain an image of the specimen surface.
Concerning the “pulsed-force-mode” microscopy reference is hereby made to:    Rosa A, Weiland E, Hild S and Marti O, The simultaneous measurement of elastic, electrostatic and adhesive properties by scanning force microscopy; “pulsed-force-mode-operation”. Meas. Sci. Technol. 8, (1997), 1-6whose disclosure is fully included in the present application by reference.
The disadvantageous aspect in the imaging method of a specimen with the help of the “pulsed-force” microscopy as has become known from the state of the art, e.g. through the above document, is that triggers need to be placed for processing the analog signals. Since the evaluation of the pulsed-force curve is substantially limited to the time of the occurrence of the triggers, this leads to inaccuracies and optimal signals can only be obtained with difficulty.
Moreover, the setting of the triggers requires skilled staff and is very time-consuming.
A further disadvantage in the “pulsed-force” microscopy according to the current state of the art is that the possibilities for evaluating the force curves is very limited and thus remain inaccurate. As a result, it is possible to obtain quantitative measured values which are relevant from a viewpoint of material sciences only with much difficulty.
Moreover, the method is limited to a maximum of two variables which can be evaluated per measurement made with scanning microscopy, which is often inadequate.
Other embodiments of scanning probe microscope comprises at least one peripheral electrical component, such as for instance a D/A or A/D converter and a programmable logic device. One particular programmable logic device is a so-called programmable gate array (FPGA). Preferably, in the present invention the programmable logic device is developed as an FPGA.
The scanning probe microscope preferably serves to generate the image of a surface of a sample being analyzed. In images of this kind the resolution can be better than 1 μm lateral to the surface and better than 100 mm perpendicular to the surface. The scanning probe scans the surface point-by-point. In one application, which serves only as an example and is not to be seen as limiting the scope of the invention in any way, the distance between the scanning probe and the sample surface is periodically modulated, such that for instance a force-time curve results. This force-time curve can then be evaluated and the development of different surface parameters can be ascertained. A scanning probe can also be an optical probe for directing light onto a surface to be scanned, for instance by means of a cantilever tip into which laser light is coupled. With respect to near field scanning optical microscopy reference is made to U.S. Pat. No. 5,756,977, DE 19902235 A1 and DE 19902234 A1. A further form of optical scanning probe microscopy is scanning probe Raman spectroscopy/microscopy.
Consequently, in the present application scanning probe microscopy is very generally defined as a microscope whereby a sample surface is scanned point-by-point. Therefore, this definition of the scanning probe microscope and scanning probe microscopy include for instance confocal microscopy, scanning force microscopy, scanning tunneling microscopy, optical near-field microscopy and also scanning electron microscopy. Accordingly, scanning probe microscopy is not restricted to scanning probe microscopy involving a force-time curve being recorded with the aid of a scanning probe. Consequently, the description of the invention for a scanning probe microscope for recording force-time curves only serves to describe the invention better by means of an example, but in no way restricts the scope of the invention. Scanning probe microscopy for recording force-time curves is just a preferred embodiment and is not a restriction.
If in a scanning probe microscope as in the state of the art, the peripheral components are actually controlled by means of controllers or microcomputers via a data bus system, such a transfer of data by means of a bus system has numerous disadvantages.
As the controllers or the digital signal processors (DSP) control the peripheral devices, such as for instance the D/A converters, the A/D converters or the digital input and output devices via a bus or a plurality of buses, developed as multi-drop buses, these must be provided with electronic digital technology for controlling access to the bus of for instance the controller or the microprocessor and for decoding the requests from the microcontroller or the digital signal processor. In addition, synchronizing the analog/digital converters and/or the digital/analog converters and the digital input and output devices requires further logic components in order for instance to synchronize a scanning movement involving the simultaneous movement of more than one channel and to simultaneously measure the values for each scanning point. Furthermore, the circuits require complex terminals to maintain the integrity of the data.
WO2004/057303 makes known a controller for a scanning probe microscope that employs a programmable logic device in the form of FPGAs. However, in WO2004/057303 it is not the entire programmable logic control unit that is developed as a programmable logic device, but just one part, namely the lock-in amplifier, developed in the programmable logic device as a digital two-phase lock-in amplifier. This fully digital programmable logic lock-in amplifier is connected to a digital signal processor that, in turn, is part of the controller. The programmable logic device in WO2004/057303 in the form of an FPGA is only an auxiliary component of the digital signal processor for signal processing and not the controller itself. Consequently, the signal processor in WO2004/057303 encompasses the scanning probe microscope controller for controlling the control circuits and the entire scanning probe microscope scan and not the programmable logic. The digital signal processor communicates with all peripheral devices by means of a multi-drop bus.
A further disadvantage of these kinds of systems in accordance with the state of the art is their inflexibility. If one wishes to add a new state-of-the-art peripheral device to the system it is necessary to calibrate the bus timing again as the additional data load resulting from the system automatically puts an extra load on the bus system. The bottleneck caused by this bus system also constitutes a bottleneck for the digital signal processor or the microcontroller as only one command or just a few parallel commands can ever be executed in the microcontroller or processor. Furthermore, every peripheral device in a state-of-the-art system with a multi-drop bus is constructed in a very complex manner as parts of the peripheral component are required to implement the bus protocol used by the multi-drop bus, for instance address decoding or bus arbitration. Furthermore, circuit termination for more than two circuits for a bus system is very complex and elaborate.